Updated

A man accused of killing his girlfriend by dragging her behind a car with a tow strap around her neck pleaded guilty Thursday to first-degree murder, accepting a deal that spared him a possible death sentence.

District Judge Paul King immediately sentenced Jose Luis Rubi-Nava, 38, to life in prison without parole for the September 2006 death of Luz Maria Franco Fierros, 49.

Prosecutors were seeking the death penalty but agreed to the life sentence if Rubi-Nava pleaded guilty to murder. They also agreed to drop kidnapping and other charges.

Franco Fierros' battered and disfigured body was found near Castle Rock, about 20 miles south of Denver. An autopsy found that she died of strangulation and massive head wounds.

Investigators said Rubi-Nava told them Franco-Fierros was alive when he began dragging her.

A Colorado Bureau of Investigation agent testified in 2007 that Rubi-Nava told him that he and Franco Fierros had a violent argument hours before her death, and that Rubi-Nava accused her of seeing other men.

"All he had to do was leave Ms. Franco Fierros," Deputy District Attorney Leslie Hansen said Thursday. "But he couldn't do that. He wanted her to pay because he thought that she was showing interest in another man."

Hansen also said Rubi-Nava had a wife in Mexico and another girlfriend in the United States.

Authorities said both Rubi-Nava and Franco Fierros were Mexican immigrants but that he was in the country illegally.

Franco Fierros' daughter, Blanca Anel Leyva-Franco, listened to the proceedings by phone from Mexico and told the judge through an interpreter that "we really are nobody to decide what happens to other people's lives. We just want you to apply the law and do what the law requires so it doesn't happen to anybody else."

District Attorney Carol Chamber said in a written statement that she consulted with Franco Fierros' family and investigators before agreeing to the plea deal.

She said the agreement gave Rubi-Nava "a limited opportunity to confess responsibility."

Rubi-Nava's attorneys had sought to have him declared mentally retarded, which would have barred his execution under Colorado law, but a judge ruled against them.

The plea agreement also required Rubi-Nava to pay $9,200 in restitution and $91,000 in prosecution costs.